Answer:
(Please read explanation before continuing)
1. Talk about what you want to accomplish.
2. Say what you want to change about yourself.
3. Say things that you hoped happened.
4. Describe how your life right then is.
5. Tell yourself what and who you want to be.
Explanation:
Now, specifics are not my strong-suit, but nor is it my weakest. Now, writing a letter to your future self is difficult, but it is simple, too. Just say what you want to accomplish, or what you've done. What you want to do. It's just like having a conversation with myself.